Alternative pre-messenger RNA splicing is a critical means of eukaryotic gene regulation that allows a single gene to produce a variety of mRNAs and proteins. Many proteins important for neuronal development and activity are functionally diversified through the differential inclusion of alternative exons. In spite of its importance to neuronal function and disease, the mechanisms controlling alternative splicing are poorly understood. We propose to continue our studies of neuronal exon splicing as mediated by the two regulatory proteins: Polypyrimidine Tract Binding Protein (PTB) and its neuronal homolog nPTB. We have shown that these proteins control large overlapping sets of splicing events during neuronal differentiation, with some exons affected by both proteins and some exons controlled only by PTB. Although most often studied as splicing repressors, PTB and perhaps nPTB can also activate some exons for splicing. We will now examine the mechanistic basis of PTB and nPTB function. We will analyze the differential targeting of exons by PTB and nPTB, and study how they can affect splicing both positively and negatively. Using an in vitro system that reconstructs the regulation of the neuron-specific N1 exon of c-src, we will analyze how PTB blocks spliceosome assembly on its target exons, and identify its important interactions within repressed exon complexes. A particular focus will be a newly identified PTB interaction with the U1 snRNA. Pre-mRNPs, within which splicing regulators function, are very large and complex. We will develop new methods for analyzing these pre-mRNPs both in vitro and in vivo. We will also complete a statistical model of PTB binding and regulation across the genome and extend this analysis to nPTB. Through these experiments, we hope to understand both the mechanisms of PTB and nPTB action, and the role these proteins play in neuronal cell biology.